The regulation of transmitter release at retinal bipolar cell terminals is important for visual signal processing from the outer retina to the inner retina in vertebrates. Although the central role of Ca2+ in transmitter release at presynaptic terminals is well established, the dynamic regulation of intracellular Ca2+ concentrations ([Ca2+]i) at presynaptic terminals and its effect on transmitter release is still largely unclear. The objective of this proposal is to study the roles of Ca2+ channels and GABA receptors in the regulation of Ca2+ dynamics at the axon terminal of mammalian bipolar cells. Experiments will be performed in solitary rat and mouse bipolar cells. Ca2+-sensitive fluorescence dyes will be used. Fluorescence signals will be monitored by a high resolution confocal microscope based imaging system. Cell's membrane potential will be controlled by patch-clamp methods and, alternatively, cell depolarization will be evoked by high K+. The specific aims are: I) to determine the spatial distribution of different types of Ca2+ channels in bipolar cells; 2) to characterize the Ca2+ transients at bipolar cell terminals during the activation of different Ca2+ channels and during cell depolarization that mimics bipolar light response waveforms; 3) to characterize the distinct effects of GABA A and GABA C receptors on the regulation of the Ca2+ transients at bipolar cell terminals; 4) to determine the spatial distribution of GABA receptors and the possible co-localization of specific types of GABA receptors with specific type(s) of Ca2+ channels. The knowledge we gain from these studies will not only lead to a better understanding of basic visual signal processing in mammalian retina but will also contribute to our knowledge of the roles of Ca2+ and inhibitory neurotransmitters in synaptic transmission in the CNS.